연습문제 3.3

(define (make-account balance password)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))

  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)

  (define (dispatch pass m)
    (cond ((eq? password pass)
           (cond ((eq? m 'withdraw) withdraw)
              ((eq? m 'deposit) deposit)
              (else (error "Unknown request -- MAKE-ACCOUNT" m))))
          (else (error "Incorect password" pass))))

  dispatch)